The present invention relates to a spark plug including a protective coat formed on a surface of a metallic member, according to which the protective coat includes a galvanized film formed on the surface of this metallic member and a hexavalent chromium-free chromate film is successively laminated on this galvanized film.
In general, the spark plug includes a metallic housing, an insulator fixed in the metallic housing, a center electrode fixed in the insulator, and a ground electrode opposed to the center electrode via a spark discharge gap.
According to this spark plug, a corrosion resisting protective coat is formed on a surface of a metallic member, such as a metallic housing or a gasket provided around the outer surface of this metallic housing (for example, refer to the Japanese Patent Application Laid-open No. 2000-252042 corresponding to the U.S. Pat. No. 6,236,148.
This protective coat includes a galvanized film provided on the surface of the metallic member and a chromate film successively laminated on the galvanized film. The chromate film is hexavalent chromium-free and contains trivalent chromium as a major component.
This chromate film is a replacement for a conventionally used chromate film containing hexavalent chromium which is known as a substance giving adverse influence to the environment.
According to this protective coat, the chromate film has a sufficient film thickness of 0.2 μm to 0.5 μm to assure excellent corrosion resistance against acid. Furthermore, this chromate film contains substantially no hexavalent chromium and is preferable in view of protection of the environment.
However, according to the inventors of this invention, it is experimentally confirmed that a conventional protective coat is subjected to exfoliations of the chromate film at a tightening screwed portion of the metallic housing during an installation work because the chromate film has a large film thickness of 0.2 μm to 0.5 μm.
Furthermore, a gasket has a folded shape so that it can be fitted into a proximal end of the tightening screwed portion around an outer cylindrical surface of the metallic housing. As the gasket is subjected to a significant bending stress, the chromate film will cause exfoliations or cracks due to this bending stress. The corrosion resistance will be lessened.
To solve this problem, it may be possible to reduce the film thickness of the chromate film so that exfoliations or cracks causing in the film under a bending stress or the like can be suppressed.
However, a thin chromate film will be relatively corrosive when it is damaged. Furthermore, there is the tendency that a hard chromate film causes exfoliations or cracks.